1. Phosphofructokinase was isolated, and partially purified by ammonium sulphate fractionation, from the fat body and flight muscle of the desert locust. 2. Ammonium sulphate appears to stabilize the enzymes, but does not activate them. 3. Both flight-muscle and fat-body enzymes give sigmoidal hexose monophosphate concentration–activity curves, which are characteristic of regulatory enzymes. 4. At low ATP concentrations both the enzyme activities increase rapidly with increasing ATP concentrations, but above an optimum concentration ATP becomes inhibitory. This optimum concentration is 0·2mm for the fat-body enzyme and 0·1mm for the flight-muscle enzyme. 5. AMP activates both the enzymes; half-maximal activation occurs at 10μm in each case, the effect being independent of substrate concentration. 6. 3′,5′-(cyclic)-AMP (0·5mm) and Pi (1mm) activate the flight-muscle enzyme, but have no effect on the fat-body enzyme. 7. FDP (1mm) inhibits both enzymes, and with the flight-muscle enzyme this inhibition is increased by increasing the ATP concentration. 8. Citrate, phosphoenolpyruvate and α-glycerophosphate have no effect on either enzyme under the assay conditions used. 9. The properties of phosphofructokinases from the locust are compared with those of phosphofructokinases from other sources.

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